I have had a recent run on failing (or failed) UPS batteries. One site has these fitted to all of their workstations and server. I went through and inspected a couple of the batteries in the workstation UPS and found then to be leaking, swollen (so much so that it was very hard to get them out of the case), cracked and leaking.
I replaced all the workstation batteries and inspected the server UPS batteries and found them to look OK. Go forward a couple of months and their server was turning itself off randomly. A thorough check on the server itself yielded nothing but when I was talking to one of the people that work in the same room as the server, he said that there was a number of clicking sounds from the server area (also where the UPS was). I then rebooted the server and whilst it was still in the BIOS, turned the power switch off for the UPS, The server also turned off so I ended up replacing the batteries and all issues with it randomly turning off are now sorted.
I then had another couple of sites that also had problems with the server UPS batteries which were replaced.
Moral of the story, these batteries have a hard life and technically should be replaced around 3 – 5 years after being put into service.